148 GEOLOGICAL BIOLOGY. 



There are examples in the class Gastropoda of orders, all 

 the members of which are restricted to a narrow and particu- 

 lar set of environmental conditions, as the Heteropods 

 (pelagic), and the Pulmonata (above tide-level). 



There are other cases in which the structural adaptation is 

 of subordinal rank, as the Ptenoglossa (pelagic) among the 

 Ctenobranchia ; and still others in which members of the sub- 

 order are found under all kinds of environmental conditions; 

 but certain families are restricted in distribution, as among 

 the Taenioglossa the family Paludinidae are all fresh-water 

 species, the Strombidae all marine, the Cyclostomidae all are 

 air-breathing and land forms. 



Again, among the members of a family there are genera 

 wdiich are restricted in their distribution to particular condi- 

 tions of environment, and other genera distributed over a 

 wider range of conditions, as in the Buccinidse Buccinum is 

 distributed in cold waters, and thus about the northern and 

 southern poles ; and Phos is restricted to warm seas, and 

 thus near the equatorial zone. 



And, to proceed one step further, particular species, of a 

 genus which is known to be distributed in all oceans, are gen- 

 erally restricted to living in a narrow range of environmental 

 conditions, to a particular limit of depth, to a particular zone 

 of temperature, and often to a particular geographical position 

 along one side of a continent or along the shores of a particu- 

 lar sea or gulf or island. 



While, however, there is this great variation in the close- 

 ness of adaptation of structure to conditions of environment, 

 it is a general law that tJic higher the taxononiic rank of a 

 group of animals the greater is the range of environmental 

 differences to ivhieh its members are adjusted ; viz., the mem- 

 bers of a family, as a rule, are distributed more widely and 

 under more diverse conditions of environment than the mem- 

 bers of some particular genus of the family, or than a par- 

 ticular species of the genus. 



